r - How to display strip labels below the plot when faceting?

Question

It seems that the strips are always above the plot created by ggplot2. Can they be moved below the plot?

For example:

library(ggplot2) 
qplot(hwy, cty, data = mpg) + facet_grid( . ~ manufacturer)

displays the car information on top. Can they be displayed be at the bottom?

Answer 1

Update: Using ggplot2 version 2.1.0, consider using switch = 'x'. See ?facet_grid for details.

Using gtable functions, it is easy to move the strip. (Or see here for anther version - swapping x-axis and strip)

library(ggplot2)
library(gtable)
library(grid)

p <- ggplot(mpg, aes(hwy, cty)) + geom_point() + facet_grid( . ~ manufacturer) +
     theme(strip.text.x = element_text(angle = 90, vjust = 1),
           strip.background = element_rect(fill = NA))

# Convert the plot to a grob
gt <- ggplotGrob(p)

# Get the positions of the panels in the layout: t = top, l = left, ...
panels <-c(subset(gt$layout, grepl("panel", gt$layout$name), select = t:r))

# Add a row below the x-axis tick mark labels,
# the same height as the strip
gt = gtable_add_rows(gt, gt$height[min(panels$t)-1], max(panels$b) + 2)

# Get the strip grob
stripGrob = gtable_filter(gt, "strip-t")

# Insert the strip grob into the new row
gt = gtable_add_grob(gt, stripGrob, t = max(panels$b) + 3, l = min(panels$l), r = max(panels$r))

# remove the old strip
gt = gt[-(min(panels$t)-1), ]

grid.newpage()
grid.draw(gt)